Strange but true: cutworms in soybeans

Black cutworms are usually associated with damage to corn. But this year, cutworms are causing significant stand loss in some soybean fields throughout central and southern Iowa. I heard numerous reports of "hundreds of acres" being sprayed and some fields were replanted because of nearly complete stand destruction. The photographs illustrate 90-95 percent stand loss in a couple of V2- and V3-stage soybean fields near Farnhamville in Calhoun County. Carroll Olsen, field specialist in crops in southwest Iowa, noted that he has seen cutworms cause substantial damage to soybeans only twice in the last 20 years. Several other seasoned agronomists also noted that seeing cutworms in soybeans was a first for them.

Seedling soybeans cut by black cutworms.

Many people commented that apparently the cutworms didn't "read the book" this year. There are several "books," but I will refer to the Handbook of Soybean Insect Pests published by the Entomological Society of America. It states: "The black cutworm is an unusual pest of soybean. Infestations in soybean typically occur in years when high numbers are present in corn. Although black cutworm is the most common species to injure soybean, other cutworms, such as variegated, may (rarely) cause injury." I would gauge the year for black cutworms in corn as a normal year; a few problems here and there, but no serious outbreaks. The cutworm situation in soybeans did not parallel that in corn but just exploded in some soybean fields.

The cutworm problem seems to have run its course, based on statewide reports from the extension field specialists in crops, so I would expect very little damage to soybeans after the end of June. Also, if the cutworms I found in Calhoun County, which were 1.5 inches in length, are representative of cutworms across the state, then they are nearly done feeding.

Black cutworms reduced this soybean population by 90-95 percent.

There are several deficiencies in our knowledge of black cutworms in soybeans. I have not been able to determine why adult female cutworms were attracted to some fields where they lay their eggs in large numbers, but they essentially ignored other fields. Preplant tillage may have an influence on egg-laying behavior. There are also no economic thresholds for black cutworms in soybeans. Treatment decisions must be based on the size of the remaining plant population and the size of the cutworms. Can we expect black cutworms to damage soybeans with increasing frequency in future years? I don't know. But, there is one thing that I do know: more people will be scouting their soybean fields next year.

If you would like more information on soybean insects, consider ordering a copy of the Handbook of Soybean Insect Pests. Call the Entomological Society of America office at 301-731-4535 or fax 301-731-4538.

This article originally appeared on pages 129-130 of the IC-482(17) -- July 5, 1999 issue.